Dislocation Source and Pile-up in a Twinning-induced Plasticity Steel at High-Cycle Fatigue

Dislocation behaviour of a twinning-induced plasticity(TWIP) steel subjected to high-cycle fatigue tests is investigated in the present study. Grain boundaries are the important sources of dislocation generation during fatigue tests, contributing to the increase in dislocation density. Continuous emission of dislocations from grain boundaries is observed in many grains. Inclusions can sustain large dislocation pile-ups at the inclusion interfaces, leading to a high stress concentration and therefore acting as potential sites of microcrack nucleation. In contrast, annealing twin boundaries are relatively weak boundaries for dislocation pile-ups. When the number of dislocations in a pile-up is large, dislocations can crossover twin boundaries and glide inside the annealing twins. The stress concentration at the twin boundary is relatively low so that twin boundaries could not act as the sites for microcrack initiation.     

Deformation twins in nanocrystalline body-centered cubic Mo as predicted by molecular dynamics simulations

This work studies deformation twins in nanocrystalline body-centered cubic Mo, including the nucleation and growth mechanisms as well as their effects on ductility, through molecular dynamics simulations. The deformation processes of nanocrystalline Mo are simulated using a columnar grain model with three different orientations. The deformation mechanisms identified, including dislocation slip, grain-boundary-mediated plasticity, deformation twins and martensitic transformation, are in agreement with previous studies. In 〈1 1 0〉 columnar grains, the deformation is dominated by twinning, which nucleates primarily from the grain boundaries by successive emission of twinning partials and thickens by jog nucleation in the grain interiors. Upon arrest by a grain boundary, the twin may either produce continuous plastic strain across the grain boundary by activating compatible twinning/slip systems or result in intergranular failure in the absence of compatible twinning/slip systems in the neighboring grain. Multiple twinning systems can be activated in the same grain, and the competition between them favors those capable of producing continuous deformation across the grain boundary.     

晶界对近片层TiAl高温动态力学行为的数值模拟

基于晶体塑性理论,给出了同时考虑位错滑移、形变孪晶和晶界变形的近片层组织TiAl本构模型;在此基础上,建立基于Voronoi算法的近层片TiAl三维多晶有限元模型,并在晶粒交界处引入壳单元来描述晶界;利用上述有限元模型,对不同温度(室温、500和700℃)和不同拉伸应变率(10-3、320、800和1 350 s-1)下近层片TiAl的塑性力学行为进行数值模拟。结果显示:模拟得到的应力塑性应变曲线与试验结果吻合较好,能够反映近层片TiAl在不同温度和应变率下的材料响应;由于晶界的存在,晶粒内的应力分布会发生明显改变,晶界附近产生一定的应力集中。此外,晶界对孪晶存在一定的阻碍作用,使得晶界附近实体单元的孪晶体积分数要略低于多晶整体的平均孪晶体积分数。     

热变形低碳钢中奥氏体静态再结晶介观尺度模拟

采用晶体塑性有限元（CPFEM）和元胞自动机（CA）耦合的方法模拟了热变形低碳钢的静态再结晶．CPFEM的计算结果定量描述了介观尺度上奥氏体变形储能的不均匀分布,为模拟再结晶的形核和长大提供了依据,从而在再结晶CA模型中考虑了不均匀变形的影响．模拟结果显示：变形储能分布不均匀使得再结晶在不同位置的形核密度不同,形核集中在晶界以及晶内储存能较大的区域;随着临界形核储能的降低,形核数量增加,再结晶晶核的位置分布趋于均匀．对不同形核判据下的再结晶动力学也作了讨论．     

Three-dimensional investigation of grain boundary–twin interactions in a Mg AZ31 alloy by electron backscatter diffraction and continuum modeling

The effect of grain boundary (GB) misorientation (θ) on twinning in a Mg AZ31 alloy is investigated using a three-dimensional (3-D) experimental and modeling approach, in which 3-D electron backscattered diffraction is performed in a volume consisting of a central grain, favorably oriented for twinning, and surrounded by three boundaries, with θ ranging from 15° to 64°. This study corroborates previous observations that twin nucleation and propagation are favored at low θ. Furthermore, it reveals that non-Schmid effects, such as the activation of low Schmid factor (SF) variants or of double tensile twins, are absent in the vicinity of low misorientation boundaries and that they become more abundant as θ increases. The 3-D morphology of individual twin variants is found to be related to their SF. High SF variants have well-established plate morphology, while low SF variants adopt irregular shapes. A crystal plasticity continuum model recently proposed by the authors is used in a very high intragrain resolution and large-scale finite element polycrystalline aggregate model of the experimental specimen. This model is shown to successfully capture the influence of θ on twin propagation and variant selection. It ultimately predicts (i) a rise in local non-basal slip with increasing θ, (ii) that low θ GB favor twin nucleation by non-Schmid stress concentrations, but that propagation is immediately accommodated by the macroscopic stress, and (iii) that high θ GB are not favorable twin nucleation sites, despite having high von Mises stress concentrations.     

Application of ICME to Engineer Fatigue-Resistant Ni-Base Superalloys Microstructures

Critical rotating components used in the hot section of gas turbine engines are subject to cyclic loading conditions during operation, and the life of these structures is governed by their ability to resist fatigue. Since it is well known that microstructural parameters, such as grain size, can significantly influence the fatigue behavior of the material, the conventional processes involved with the manufacture of these structures are carefully controlled in an effort to engineer the resulting microstructure. For a commercial Ni-base superalloy, RR1000, the development of process models and deformation mechanism maps has enabled not only control of the resultant grain size but also the ability to tailor and manipulate the resulting grain boundary character distribution. The increased level of microstructural control was coupled with a physics-based fatigue model to form an integrated computational materials engineering framework that was used to guide the design of damage-tolerant microstructures. Simulations from a 3D crystal plasticity finite element model were used to identify microstructural features associated with strain localization during cyclic loading and to guide the design of polycrystalline microstructures optimized for fatigue resistance. Conventionally processed and grain boundary engineered forgings of a commercial Ni-based superalloy, RR1000, were produced to validate the design methodology. For nominally equivalent grain sizes, high-resolution strain maps generated via digital image correlation confirmed that the high density of twin boundaries in the grain boundary engineered material were desirable microstructural features as they contribute to limiting the overall length of persistent slip bands that often serve as precursors for the nucleation of fatigue cracks.     

Simulations of stress-induced twinning and de-twinning: A phase field model

Twinning in certain metals or under certain conditions is a major plastic deformation mode. Here we present a phase field model to describe twin formation and evolution in a polycrystalline fcc metal under loading and unloading. The model assumes that twin nucleation, growth and de-twinning is a process of partial dislocation nucleation and slip on successive habit planes. Stacking fault energies, energy pathways (γ surfaces), critical shear stresses for the formation of stacking faults and dislocation core energies are used to construct the thermodynamic model. The simulation results demonstrate that the model is able to predict the nucleation of twins and partial dislocations, as well as the morphology of the twin nuclei, and to reasonably describe twin growth and interaction. The twin microstructures at grain boundaries are in agreement with experimental observation. It was found that de-twinning occurs during unloading in the simulations, however, a strong dependence of twin structure evolution on loading history was observed.     

Deformation of hierarchically twinned martensite

Shape-memory alloys deform via the reorganization of a hierarchically twinned microstructure. Twin boundaries themselves present obstacles for twin boundary motion. In spite of a high density of obstacles, twinning stresses of Ni–Mn–Ga Heusler alloys are very low. Neither atomistic nor dislocation-based models account for such low yield stresses. Twinning mechanisms are studied here on a mesoscopic length scale making use of the disclination theory. In a first approach, a strictly periodic twin pattern containing periodic disclination walls with optimally screened stress fields is considered. Strict periodicity implies that the twin microstructure reorganizes homogeneously. In a second approach, a discontinuity of the fraction of secondary twins is introduced and modeled as a disclination dipole. The stress required for nucleation of this discontinuity is larger than the stress required for homogeneous reorganization. However, once the dipole is formed, it can move under a much smaller stress in agreement with experimental findings.     

Recrystallization During Thermo-Mechanical Fatigue of Two High-Generation Ni-Based Single Crystal SuperalloysEI北大核心CSCD

通过SEM和TEM等手段研究了经热机械疲劳变形后的第三代和第四代单晶高温合金的显微组织,了解高温合金在近服役条件下的变形组织,分析单晶高温合金近服役条件下的变形机制。结果表明,第三代和第四代单晶高温合金样品中在不同{111}面上产生了大量的变形孪晶,且在平行的孪晶片层中或者孪晶片层交截周围发现大量再结晶晶粒。再结晶晶粒的界面主要由变形后的孪晶界、小角度晶界以及孪晶相交产生的大角度晶界组成。借助像差校正透射电镜解析了变形后的孪晶界结构以及孪晶诱发动态再结晶的过程,揭示了单晶高温合金热机械疲劳断裂机制。     

TWIP钢的动态力学性能及变形机制研究

利用Zwick/Roell Z100万能材料试验机和Hopkinson拉杆对TWIP钢进行了准静态及动态力学性能的研究。基于力学实验结果,修正了Johnson-Cook动态本构模型中应变硬化项以及应变强化项。采用X射线衍射(XRD)、扫描电镜(SEM)、电子背散射衍射(EBSD)技术对TWIP钢拉伸变形前后的组织进行了观察与分析。结果表明：TWIP钢在准静态加载下表现为负应变率敏感性,动态加载时表现为正应变率敏感性。拉伸过程中,孪生诱发塑性是TWIP钢的主要变形机制,同时滑移也起到重要作用;动态加载下TWIP钢中形变孪晶的起始应变和孪晶体积分数均小于准静态加载过程;形变孪晶的生成以及孪晶相互作用导致的晶粒细化,使TWIP钢兼具高强度、高塑性及高动态吸能性能,在抗冲击、抗爆领域具有广泛的应用前景。     

Tensile properties of nanocrystalline tantalum from molecular dynamics simulations

The tensile behaviors of nanocrystalline tantalum are studied using molecular dynamics simulations. The results show that the elastic modulus increases linearly with density. The flow stress decreases with decreased grain size, but increases with increased strain rate or decreased temperature. A strain rate sensitivity of ∼0.14 is derived from the simulations with a resultant activation volume of ∼1b³ associated with plastic deformation. Grain rotation, grain boundary sliding or migration, dislocation motion and intergranular activities are observed in the deformation process. Twinning is regarded to be a secondary mechanism. Stress-induced phase transitions from body-centered cubic to face-centered cubic (fcc) and hexagonal close-packed (hcp) structures take place locally, and the hcp structure is a derivative of the fcc structure. The higher the strain rate, the further delayed the phase transition. Such phase transitions are found to occur only at relatively low-temperatures and are reversible with respect to stress.     

Grain Boundary Contributions to Hydrogen-Affected Plasticity in Ni-201

Hydrogen embrittlement of structural materials, such as nickel-based alloys, is often characterized by enhanced dislocation processes as well as grain boundary decohesion leading to macroscale intergranular fracture. Nanoindentation and scanning probe microscopy (SPM) were used to characterize slip transfer across random grain boundaries and Σ3 recrystallization twins in annealed Ni-201. Thermal hydrogen charging leads to an increase in slip step width within pileups produced by nanoindentation along grain boundaries. The likelihood of slip transmission in the presence of hydrogen depends on the ease of slip within adjacent grains as well as on the misorientation of the grain boundary between them. The observed changes suggest that hydrogen limits dislocation cross-slip while increasing overall dislocation mobility. Coupled nanoindentation and SPM investigations provide a unique, local method for analyzing hydrogen effects on dislocation plasticity, which will be useful in developing grain-boundary-engineered materials.     

Exploring the Formation Mechanism of Deformation Twins in CrMnFeCoNi High Entropy Alloy

The deformation twins initiated in CrMnFeCoNi high entropy alloy at cryogenic temperature are experimentally studied. Under the external loading, a three-dimensional shear stress concentration originating from dislocation tangling at both the grain boundaries and twin boundaries could be formed, which promotes emission of partial dislocations from the planar defects and is thus considered to be the key factor for twin formation. A sympathetic nucleation mechanism is proposed to describe the nucleation behaviors of twins.     

Texture analysis of the transition from slip to grain boundary sliding in a discontinuously recrystallized superplastic aluminum alloy

Texture and microtexture measurements were correlated with mechanical property data for a superplastic 5083 aluminum alloy. Prior processing had included an overaging treatment followed by severe rolling deformation and the as-received material was annealed prior to mechanical testing. Discontinuous recrystallization by particle-stimulated nucleation during the annealing accounts for a predominantly random texture, although a weak {100}<0vw> component was present, as well as a random grain boundary disorientation angle distribution. During elevated temperature deformation under dislocation-creep-controlled conditions, a distinct <111> fiber component and a relatively weak {100}<001> cube orientation, which are mutually compatible during uniaxial tensile extension, became apparent in the texture. Also, low-angle boundaries became evident in the disorientation distribution. In contrast, the random texture component and the randomness of the disorientation distribution became more evident when the material was deformed under conditions of grain boundary sliding control of deformation. A transition from dislocation creep to grain boundary sliding observed in the microtexture measurements of this work may be predicted by treating constitutive equations for dislocation creep and grain boundary sliding in an additive manner.     

A new look at the influences of load, grain size and grain boundaries on the room temperature hardness of ceramics

The measured load (size) effect on the hardness is modelled assuming that for the extension of the plastically deformed zone, growth and multiplication of pre-existing elements of plasticity are more effective than the generation of new dislocations and twins in the virgin material around. This idea also explains the decreasing load effect at smaller grain sizes. Therefore, microhardness approaches must not be used to investigate grain size effects in ceramic microstructures. The comparison of the real grain size effect in sintered Al₂O₃ with the indentation size (load) effect in sapphire single crystals (shown to simulate the grain size effect in polycrystals but avoiding the influence of grain boundaries) reveals an important contribution of the grain boundaries to the permanent deformation at the indentation site at room temperature even for coarser microstructures and rules out the chances for a strong hardness increase in oxide ceramics at grain sizes < 100 nm. However, first results indicate a possibly different behavior in binder-free carbides where the covalent character of interface bonding may reduce the degree of grain boundary deformation at room temperature as it is known to reduce the microplastic deformability of the crystal lattice.     

Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth–microstructure relationship of nanocrystalline alloys

While previous studies have reported that nanocrystalline materials exhibit poor resistance to fatigue crack growth (FCG), the electro-deposited nanocrystalline Ni–Co alloys tested in this paper show superior resistance to FCG. The high damage tolerance of our alloy is attributed to the following: alloying with Co, low internal stresses resulting in stability of the microstructure, and a combination of high strength and ductility. The high density of grain boundaries interact with the dislocations emitted from the crack tip, which impedes FCG, as predicted by the present model and measured experimentally by digital image correlation. Further, the addition of Co increases the strength of the material by refining the grain size, reducing the fraction of low angle grain boundaries, and reducing the stacking fault energy of the material, thereby increasing the prevalence of twinning. The microstructure is stabilized by minimizing the internal stress during a stress relief heat treatment following the electro-deposition process. As a result grain growth does not occur during deformation, leaving dislocation-mediated plasticity as the primary deformation mechanism. The low internal stresses and nanoscale twins preserve the ductility of the material, thereby reaching a balance between strength and ductility, which results in a superior resistance to FCG.     

Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations

Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.     

Impurity-induced embrittlement of heat-affected zone in welded Mo-based alloys

Characteristics of strength and plasticity, fracture mode and grain boundary segregation for two Mo-based alloys with different bulk compositions, recrystallized by either furnace annealing or rapid heating followed by quenching, are studied as a function of heating temperature by mechanical test, scanning electron microscopy, Auger electron spectroscopy and computer simulation. There exists an essential difference in both segregation behaviour and mechanical properties between as-annealed and as-quenched structural states. The rapid quenching causes strong oversaturation of the grain boundaries. In this case, intergranular enrichment is approximately twice as high as that in as-annealed alloys, and spontaneous nucleation of brittle microcracks is observed at certain embrittled boundaries. The proposed high-speed heat treatments are considered as a promising method for modelling of the structural states of the heat-affected zone of weldments. The results obtained are discussed from the viewpoint of possible reasons of impurity-induced embrittlement of Mo-based alloys.     

不同状态条件下AZ80镁合金微观组织演化

利用电子背散射衍射(EBSD)技术观察研究了不同状态条件下AZ80镁合金的微观组织,分析了不同状态条件下AZ80镁合金的微观组织演化。结果表明:按照铸态、均质化热处理态和塑性变形态顺序,试样平均晶粒尺寸逐渐减小,平均晶粒形状纵横比呈先增大后减小的变化趋势,网状β-Mg₁₇Al₁₂相逐渐消失,材料塑性和强度得到提高;晶界的协调作用主要受晶界迁移与几何必须位错(GND)密度两方面的影响,伴随晶界的迁移,小角度晶界逐渐增多并阻碍再结晶的进行,使再结晶区呈先增大后减小的变化趋势,而亚结构区则逐渐减小,亚结构区的消失为孪晶的形成提供了能量,促进了孪晶的形成;几何必须位错密度呈先减小后增大的变化趋势,几何必须位错密度的减小促进了孪晶的生长和晶粒间的旋转移动,而几何必须位错密度的增大则使孪晶的生长和晶粒间的旋转移动受到阻碍。塑性变形态镁合金出现了典型的基面织构。